The present invention relates to an extrusion type coater and a coating method. More specifically, the invention relates to a coater and a coating method for coating thin film stably at a high speed when producing photographic materials or magnetic recording media, and so on.
Various coating methods such as roll coating, gravure coating, extrusion coating, slide bead coating, and curtain coating are generally known.
A photographic material is produced by applying undercoating liquid onto a supporter and then photographic emulsion on it and a magnetic recording medium is produced by applying magnetic coating liquid onto a supporter. The coating method often used for photographic materials is slide bead coating, extrusion coating, curtain coating, or roll coating and the coating method often used for magnetic recording media is reverse roll coating, gravure coating, or extrusion coating. Among them, the extrusion coating method is considered to be superior to the others because the coating film thickness is determined by the liquid flow rate, and no excessive coating liquid is returned, and a uniform coating film thickness is obtained regardless of the viscosity of coating liquid.
In the aforementioned manufacture of photographic materials or magnetic recording media, there are increasing demands for thin film coating so as to cope with high density or for high speed coating as far as possible so as to increase the productivity.
The prior arts of the extrusion coating method, which are mentioned, for example, in Japanese Patent Laid-Open Nos. 57-84771, 58-104666, 58-202075, and 60-238179, are known.
Although any of the above prior arts of the extrusion coating method can produce a uniform coating film thickness, the applicable good coating condition range is narrow and under the high-speed coating condition that low-viscosity liquid such as under-coating liquid for photographic materials is applied at a speed of several tens m/min or high-viscosity liquid such as magnetic coating liquid for magnetic recording media is applied at a speed of several hundreds m/min, it is very difficult to coat thin film stably.
At the time of high-speed thin film coating mentioned above, faults such as streaks in the longitudinal direction or cross lines in the transverse direction are generated very frequently.
In view of the above problem, the inventor has devoted himself to study and found that to realize stable high-speed thin film coating, it is necessary to increase the force for applying coating liquid onto the surface of the supporter and to increase the force, it is necessary to increase pressure (generation pressure) which is generated by the above coating liquid between the back edge surface and the coating object, for example, the surface of the supporter when the coating liquid passes the back edge surface and the present invention has been achieved.